Filler for roll-weld structures

ABSTRACT

A roll-welding process for fabricating beryllium roll-welded panels where the filler material is an austenitic manganese or Hadfield steel which can be chemically or mechanically removed from the structure that has been diffusion bonded by the process.

United States Patent Anderson, Jr. et al.

[ 1 June 6,1972

[54] FILLER FOR ROLL-WELD STRUCTURES [72] Inventors: Raymond B.Anderson, Jr., Santa Ana; Richard A. Rawe; Bennett V. Whiteson, both ofGranada Hills, all of Calif.

[73] Assignee: McDonnell Douglas Corporation [22] Filed: Oct. 1, 1968[2I Appl. No.: 764,064

52 U.S.CI ..29/423,29 472.1,29 4723, 29 493 51 1nt.Cl ..B23p 17/00 58FieldofSearch ..29/423,471.1,472.1,472.3, 29/493 [56] References CitedUNITED STATES PATENTS 3,044,160 7/1962 Jafi'ee ..29/423 UX 3,345,73510/1967 Nicholls ..29/47l.l X 3,380,146 4/1968 Babel et a1. ..29/4233,419,951 l/1969 Carlson ..29/423 X 3,427,706 2/1969 Jafiee ..29/47 1.13,453,717 7/1969 Pfafi'enberger et a1. ..29/423 Primary ExaminerJohn F.Campbell Assistant Examiner-Richard Bernard Lazarus Attorney-Walter .1.Jason, Donald L. Royer and Robert 0. Richardson [57] ABSTRACT Aroll-welding process for fabricating beryllium roll-welded panels wherethe filler material is an austenitic manganese or Hadfield steel whichcan be chemically or mechanically removed from the structure that hasbeen diffusion bonded by the process.

2 Claims, 3 Drawing Figures FILLER FOR ROLL-WELD STRUCTURES BACKGROUNDOF THE INVENTION The roll-weld process is a method of combiningdifficult to weld materials such as beryllium or titanium into anautogeneous weld in the solid state by the application of heat andpressure during a hot rolling operation. The'parts to be welded arepositioned in an abutting relationship within a surrounding frame ofanother material, also of a different composition, so as to provide fora pack having substantially no void spaces. Cover sheets are welded tothe opposite sides of the frame and air evacuated from the pack. Afterthe pack is subjected to a suitable temperature and pressure duringrolling to unite the parts, the cover sheets, frame, and filler materialare removed. This roll-weld process is more fully disclosed in U. S.Pat. No. 3 ,044, 160 entitled Method of Producing Ribbed Metal SandwichStructures," by R. I. Jaffee, and issued July I7, 1962.

Problems in filler material selection have been encountered when metalssuch as titanium and beryllium have been used as parent metals in theroll-weld process. It was found that the use of mild steel fillermaterials containing up to 0. l 5 percent carbon resulted incontamination of the titanium surface. When the composition of thesurface of steel filler is less than 0.15 percent carbon, such as adecarburized steel, a strong bond between the filler and titanium parentmetal is developed during rolling, with resultant contamination of theparent metal surface. This led to the discovery that the amount ofcarbon at the surface of the steel directly affected the degree of ioncontamination and subsequently the degree of bond between the filler andparent metal. It was also discovered that carbon contents greater than0.40 percent at the tiller surface were necessary to obtain ease ofmechanical removal of the steel filler from the titanium components.Disclosure of the carbon barrier concept of steel filler preparation iscontained in U. S. Pat. No. 3,380,l46 forCarbon Barrier, issued Apr. 30,1968 to Henry W. Babel, et al.

The fabrication of roll-weld panels from other low density materialssuch as beryllium will result in composite structures with exceptionallyhigh strength-to-weight ratio. The inherent brittleness of berylliumresulting from its crystallographic structure places additionalrequirements on the selection of both filler and surrounding packmaterials which are not necessary with the more ductile parent metals.In particular, filler materials must be selected to insure that levelsof stress placed in the beryllium components during processing are notexcessive, causing rupture of the bonds and cracking of the beryllium.Such problems could arise if the filler material undergoes an allotropictransformation during processing, resulting in expansion during cooling,does not possess a coefficient of thermal expansion compatible withberyllium, and does not possess deformation characteristics similar tothose of beryllium. In addition, the filler material must notcontaminate the parent metal surface during roll-welding. Failure of thefiller material to possess these properties would result in unacceptableroll-weld panels and sever-l limits the application of the process toberyllium.

Beryllium and its alloys represent a relatively new structural materialfor use in aerospace industries. The material has long been used asreflectors or moderators in nuclear reactors but, more recently, seriousconsideration has been given to its structural applications. Animportant feature of beryllium is that it is approximately 35 percentlighter than aluminum and possesses a specific heat approximately twotimes that of aluminum. The materials elastic modulus is higher thanthat of any other structural material and persists at temperatures whereother light metals are no longer operative. In combination, theproperties of relatively high modulus and density characterize amaterial with unusual potential. The material also possesses usefulmechanical properties to temperatures above 800 F as experienced by highperformance space, aerial and certain other vehicles.

Fabrication, handling, and joining of beryllium products is difficult.Forming must be carried out at temperatures above l,200 F except forberyllium alloys containing extremely low interstital content. Joints inberyllium cannot be satisfactorily prepared by ordinary weldingtechniques. Grain growth further embrittles the joint. Adhesive bondinghas been successfully used where the joints are not excessively loadedor do not operate at high temperatures. Mechanical joints have beenconsidered to be most applicable to beryllium although they aredifficult to fabricate, lack strength, and are inefi'icient comparedwith normal welded joints. Handling and processing of beryllium must becarried out in special facilities due to the toxicity of the BeO dustand vapors. On the other hand, joints in beryllium produced byroll-welding possess strengths comparable to the parent metal as well asresulting in a joint of higher efficiency.

A basic part of the roll-weld process is the use of filler material tosupport the parent metal components during rolling. A picture frame yokeand cover sheet arrangement is used to provide a hermetically sealedenvelop around the panel. After rolling, the yokes, cover sheets, andfiller material are either chemically or mechanically removed. Inroll-welding of titanium panels, steel filler bars containing greaterthan 0.40 percent carbon are normally specified to minimize ironcontamination of the titanium and to provide for mechanical removal ofthe filler bars. It was discovered, however, that the use of carbonsteel filler material in the roll-welding of beryllium panels wasunsatisfactory because of the brittleness of the parent metal andmetallurgical changes which occur in the steel during processing. Theseallotropic transformations are obvious from a study of the iron-carbonphase diagram. Steel will transform upon heating from the body centeredcubic crystal structure to the face centered cubic structure with anaccompanying contraction in the lattice. On cooling to about 1,200 P,this face centered cubic structure undergoes the reverse transformationto the body centered cubic structure with a resultant expansion of thelattice structure. This expansion of the filler material after the panelhas been rolled to final dimensions results in high stress levels in theberyllium components. Cracking of the beryllium is the end result. The

use of steel filler with other materials such as titanium containing alower modulus of elasticity than beryllium does not produce cracking ofthe parent metal components.

SUMMARY OF THE INVENTION Elimination of allotropic transformations andtheir adverse effects on beryllium in the roll weld process can beaccomplished by selection of a filler material which does not undergosuch changes during processing temperatures, contains similardeformation characteristics as beryllium, will not contaminate theberyllium surfaces, and can be readily removed by conventional methods.The discovery was made that the use of austenitic manganese steel,sometimes referred to as Hadfields Steel, with beryllium in the rollweld process eliminates cracking of the beryllium components, results ingood beryllium-beryllium bonds, and provides a filler material withdeformation characteristics similar to beryllium and one which can bereadily removed after processing. The austenitic manganese steelnormally contains 1.2 percent carbon and 12 to 15 percent manganese asthe essential elements. The material is similar to other austeniticsteels in that it remains face centered cubic in structure through allprocessing temperatures encountered in beryllium roll-welding. Thecoefficient of thermal expansion is quite similar to that of austeniticstainless steel and compatible with that of beryllium. Its deformationcharacteristics are similar to those of beryllium at rollweldtemperatures. No evidence of contamination of the beryllium surface hasbeen discovered with the use of austenitic manganese steel as fillermaterial.

Removal of the filler material from the beryllium roll-weld panel mustbe accomplished by chemical leaching because of the brittlenessexhibited by beryllium. Mechanical removal is extremely hazardous withberyllium, resulting in high impact and vibrational loads. Under suchconditions, cracking of the beryllium components during. removal wouldbe inevitable. Chemical leaching of the austenitic manganese steelfiller can be accomplished readily with the use of a solution of nitricacid in concentrations of 50 percent or greater. The beryllium panelincluding the filler material can be submerged in the solution and leftuntil the filler material has been leached out. It was discovered thatmore diluteacid concentrations resulted in an attack on the berylliumduring filler removal.

The principal advantage in the use of the present invention is theability to fabricate a beryllium composite structure wherein removal ofthe filler material from the beryllium rollweld-structure is donewithout damage to the parent metal. Satisfactory removal will result ina beryllium structure in which the parent metal components have beenstrongly rollwelded together with no contamination or cracking of theparent metal components resulting from filler material interaction orincompatibility.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view showing therelationship of the various elements comprising a rollweld pack used inthe practice of the present invention;

FIG. 2 is an enlarged sectional view of a beryllium panel before thefiller material has been removed; and I FIG. 3 is a sectional view of analternate form of beryllium panel, also without the filler materialbeing removed.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS Referring now to theexploded view of the pack assembly for producing roll-weld truss coresandwich panels, as shown in FIG. I, there is shown a bottom cover plate10, top cover 12, and yoke 14 which when assembled comprises the packwithin which a composite structure may be placed for the rollweldingprocess. Yoke 14 has a thickness to separate the cover plates and 12equal to the thickness of the composite structure placed within thecentral opening 18 therein. The composite structure in this illustrativeembodiment consists of a beryllium top cover sheet 20, a berylliumbottom cover sheet 22, and beryllium ribs 24 of such length as to abutthe inner surfaces of these cover sheets. In forming a truss core, theseribs are spaced apart and are inclined approximately 70. In order tofabricate a truss core sandwich panel by the roll-welding process, allof the void spaces within the yoke 14 must be filled. Accordingly, steelfiller bars 26 are so shaped to fit between the ribs 24. The enlargedview in FIG. 2 shows this more clearly. After machining and prior toassembly, all components are chemically cleaned. To ensure properfitting in the cavity, the beryllium ribs were chemically etched. Asolution of 95cc nitric acid (70-71 percent reagent grade), 95cc water,and 23 grams of ammonium fluoride is a typical chemical milling solutionfor this purpose. The faying edges of the ribs and the inner surfaces ofthe skin sheets 20, 22 were grit blasted and then dipped in a solutionof 10 percent nitric acid 70-71 percent reagent grade), 90 percentwater, rinsed in warm water, dipped in a 10 percent sulphuric acid (95.5to 96.5 percent reagent grade) 90 percent water solution, again rinsedin warm water and finally rinsed twice in ethyl alcohol.

The steel components were generally degreased either in acetone ortrichloroethylene, rinsed in alcohol and annealed in a vacuum for a halfhour at l,600-l,700 F. A coating of high grade aluminum oxide applied tothe stainless steel cover plates 10 and 12 facilitates separating of thecover plates from the skin sheets 20, 22 after the bonding process hasbeen performed. After assembly, the cover plates l0, l2 and yoke 14 werewelded in an argon-helium atmosphere. After welding, the pack wasevacuated at room temperature to approximately 1 micron or less. Thepack was then heated to approximately l,525 F and subjected to a rollingpressure where its thickness was reduced by about 10 percent for eachpass. A series of passes from 4 to 8 is considered typical in reducingthe thickness as desired. After the rolling operation has beencompleted, the pack was allowed to slow cool by placing it in a furnaceoperating at 1,200 l ,300 F and then shutting off the furnace. After therolling and cooling, the weldment was cut and the stainless steel coverswere easily removed. The balance of the yoke was carefully removed fromthe panel with a cutoff wheel. Removal of the steel filler bars 26 fromthe roll bonded structures is best done by leaching in nitric acid. Thehigher the acid concentration, the higher the rate of removal of thetiller bars. Also it was observed that a weaker solution in someinstances does attack the beryllium. Accordingly, a chemical leachingsolution of from 50 to 75 percent nitric acid is preferred.

While the beryllium panel shownin FIGS. 1 and 2 was a truss coresandwich panel, other configurations are possible and in some instancesare preferable. The configuration shown in FIG. 3 consists of a bottomcover sheet 30 having vertical rib sections 32 with segmentedreinforcing strips-34 providing a T-shaped reinforcement to the bottomcover sheet 30. Filler material 36 fills the spaces between the ribsections 32 and reinforcing segments 34 in the same manner as that shownin FIGS. 1 and 2.

We claim:

1. The method of roll-welding of beryllium parts comprising the stepsof:

a. assembling the parts to be welded with sections abutting in a metalyoke,

b. filling the metal yoke with filler material of an austeniticmanganese steel with a nominal composition of approximately 1.2 percentcarbon and l2-l 3 percent manganese,

c. covering the parts and filler material with metal cover plates,

d. withdrawing the air therefrom,

e. heating and rolling the metal yoke and its contents until anautogenous weld is made between the abutting surfaces of the berylliumcomponents, and

f. removing the filler material, yoke and cover plates from theberyllium parts.

2. In the method set forth in claim 1, the improvement of:

employing a chemical leaching solution of 50-75 percent nitric acid toremove the austenitic manganese steel filler material from the berylliumcomponents thereby eliminating the isolated attack on the berylliumcomponents associated with the use of more dilute acid solutions.

2. In the method set forth in claim 1, the improvement of: employing a chemical leaching solution of 50-75 percent nitric acid to remove the austenitic manganese steel filler material from the beryllium components thereby eliminating the isolated attack on the beryllium components associated with the use of more dilute acid solutions. 